The field of invention is moisture absorption media regeneration, more particularly continuous regeneration of moisture absorbing ceramic materials using microwave energy.
Dry gas is necessary in many scientific and commercial applications, such as glove boxes where moisture or other gaseous state contaminant must be removed from a recirculating gas. Commonly, the recirculating gas is exposed to an absorption media that has a tendency to absorb the contaminant. The absorption material absorbs contaminants, such as water, present in the gas. The gas having a lowered contaminant level is then recirculated back to the glove box or other process. After a period of use, the absorption material reaches its maximum absorption level and ceases to efficiently absorb additional contaminants. The material must then be replaced or regenerated to continue cleansing the recirculating gas.
Molecular sieve materials, such as calcium aluminum silicates and similar zeolites, in small bead form having pore sizes of 3 to 13 angstroms, are used as moisture and gas scrubbers to maintain low levels of moisture in recirculating gas streams. These materials are very efficient when dry and can remove moisture down to the low part per million levels in flowing gas streams.
In a typical prior art application, moist recirculating gas is routed through a box containing moisture sieve beads. After the beads have taken up their full load of moisture, they are regenerated by heating to 250.degree. C.-300.degree. C. for several hours to drive off the absorbed moisture and gases. In a continuous process, this regeneration period is disruptive requiring that the process be shut down while the beads are being heated.
A prior art alternative to shutting down the entire process for a period of hours was developed using a two box method. In a two box method, the recirculating gas is rerouted to a second box containing absorption media while the first box is undergoing regeneration. This solution significantly reduces the down time for the continuous process.
The down time in a two box method, however, is not eliminated and other disadvantages remained. Determining when the beads are saturated or inefficient requires a humidity measurement in the gas stream and operational delays caused by switching from the moisture saturated bead bed to a regenerated bed continue to cause problems. In addition, regeneration in the prior art is typically performed by heating the absorption media by electrical resistance heaters which depend on thermal heat transfer by slow conduction and convection heat transfer modes.
In one apparatus for regenerating an absorption media, as disclosed in U.S. Pat. No. 5,509,956, Opperman et al., microwave energy is used to regenerate contaminant absorbing beads. In this apparatus, contaminated gas enters a container containing a polymeric absorption media, the media absorbs the contaminants, and then the decontaminated gas exits the container. When the media reaches a predetermined saturation level, the gas inlet and outlet are closed off, stopping the cleansing process, microwave energy is then directed into a hollow guide tube that distributes the energy throughout the entire container causing the absorption media to release the absorbed contaminants. The contaminants are then siphoned out of the container through a suction port. The application of microwave energy to a drying or decontaminant apparatus shortens the regeneration time. As in other prior art, however, the apparatus disclosed in U.S. Pat. No. 5,509,956 requires a regeneration period that disrupts a continuous flow process.
Certain ceramic compounds have desirable absorption characteristics. Ceramics, in general, however, are difficult to heat by means of microwave energy due to small dielectric loss factors as disclosed in U.S. Pat. No. 4,771,153, Fukushima et al. The ceramic compositions typically used in heating applications, as disclosed in U.S. Pat. No. 4,320,253, Fisher, are ferrite based. Ferrite based ceramics do not appreciably increase in temperature when subjected to microwave heating. As a result, the use of microwave energy to heat ceramic compositions for drying gas purposes and other uses is not known.